Electrodeposition of chromium



Patented Mar. 3, 1953 ELECTRODEPOSITION F CHROMIUM Norman Hackerman, Austin, Tex., assignor to Research Corporation, New York, N. Y., a corporation of New York No Drawing. Application December 18, 1950, Serial No. 201,487

2 Claims. (Cl. 20451) Thi invention relates to the electrodeposition of chromium.

Chromium plate electrically deposited on other metals provides a bright decorative finish and an extremely hard surface. The chromium film is very corrosion resistant since it generally is in the passive state. However, chromium electroplate alone does not protect the base metal from corrosion due to the presence of a crack network in the chromium plate, the cause of which has not been ascertained.

It has now been found that the crack network in electrodeposited chromium can be greatly reduced or eliminated by the electrodeposition of chromium from a bath containing a salt of indium. The plated metal film produced in this way has all of the advantages of chromium plate and in addition provides greatly increased protection of the base metal, such as copper or steel, against corrosion.

The indium salt may be added to the chromic acid-sulfuric acid plating solutions commonly used for chromium plating, advantageously in the form of indium sulfate. Preferably from about 14 to about 16 parts of indium sulfate should be present for each 100 parts of chromic acid in the plating solution. Plating from such solutions may be efiected at voltages of from 2.5 to 4.0 volts and at current densities of from 6 to 18 ampa/dm. at about 30 C. Preferably the voltage is maintained between about 3.2 and about 3.5. The optimum current density will vary with the concentrations of the bath components.

A hard, bright plate substantially free from cracks and providing a high protection against corrosion may be obtained by the following procedure which is illustrative of the principles of the invention:

A steel or copper article is plated from a bath containing 250 g./liter of Cl'Os, 2.5 g./liter of H2804 and 38.7 g./liter of indium sulfate at a current density of 13 to 15.5 amps/dm. and a voltage of 3.2 to 3.5 using lead anodes and continuing the plating for about twelve hours at 1 In general, in a chromium oxide-sulfuric acid plating solution of the composition given in the example, it is desirable to use at least about 17 grams per liter of indium sulfate. Improved brightness of the plate and substantial freedom from cracks is obtained with higher amounts of the indium salt of the order of 35 to 40 grams per liter.

The bright, substantially crack-free, plate is much less brittle than the chromium plate obtained in the absence of indium as shown by bend tests.

I claim:

1. The method of electrodepositing bright, smooth, adherent films of chromium upon a cathodic workpiece which comprises immersing the cathode workpiece in an aqueous solution of chromic acid and sulfuric acid containing from about 14 to about 16 parts of indium sulfate to each parts of chromic acid and passing an electric current through said solution between the cathodic workpiece and an anode immersed in said solution.

2. The method of electrodepositing bright, smooth, adherent films of chromium 'upon a cathodic workpiece which comprises immersing the cathodic workpiece in an aqueous solution of chromic acid and sulfuric acid containing from about 14 to about 16 parts of indium sulfate to each 100 parts of chromic acid and passing an electric current at a current density of from about 13 to about 15.5 amp./dm. through said solution between the cathodic workpiece and an anode immersed in said solution.

NORMAN HACKERMAN.

REFERENCES CITED The following references are of record. in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,802,463 Fink Apr. 28, 1931 2,358,029 Phillips et al. Sept. 12, 19M 

1. THE METHOD OF ELECTRODEPOSITING BRIGHT, SMOOTH, ADHERENT FILMS OF CHROMIUM UPON A CATHODIC WORKPIECE WHICH COMPRISES IMMERSING THE CATHODIC WORKPIECE IN AN AQUEOUS SOLUTION OF CHROMIC ACID AND SULFURIC ACID CONTAINING FROM ABOUT 14 TO ABOUT 16 PARTS OF INDIUM SULFATE TO EACH 100 PARTS OF CHROMIC ACID AND PASSING AN ELECTRIC CURRENT THROUGH SAID SOLUTION BETWEEN THE CATHODIC WORKPIECE AND AN ANODE IMMERSED IN SAID SOLUTION . 